Joanne Segars, chief executive of the National Association of Pension Funds, said the body “strongly supports” a new single tier state pension.

She warned: “The Government has to ensure that the changes are implemented in a way that does not damage company pension schemes.

“This is a very tight timeframe and we question whether it can be delivered. Schemes need flexibility and time to adapt.

“It would be a shame if mistakes were made in a rush to implement the changes.”

She added: “Businesses that get caught on the wrong side of these changes will lose a significant rebate from the end of contracting out, and this extra cost may prompt them to close their pensions altogether.”

Chancellor George Osborne told the Commons that the reforms will give employees a “fair deal”.

The Chancellor said that someone who is aged 40 years old when the single tier pension is introduced and who has always been contracted out will pay an extra £6,000 in National Insurance over the rest of their working life, and in return they will get an extra £24,000 in state pension over the course of their retirement.

The reforms spell an end to "contracting out" of occupational pension schemes, which saw employees and employers on final salary pensions opt out of the second state pension, instead paying their money into their pension scheme while making reduced National Insurance contributions.

Mr Osborne said that the early introduction of the £144 payment would "help the low paid, the self-employed and millions of women most of all".

But unions said the Chancellor's decision to accelerate the introduction of the flat rate state pension was a "cash grab on pensions", after it emerged it will bring an extra £5billion for the Treasury.

This is because all employees in defined benefit schemes, where payments are guaranteed, would pay higher national insurance contributions from 2016.